London, July 2 (Reuters): Computers could soon erect invisible barriers in the sky to protect cities against attack by hijacked planes.

Researchers at the University of California at Berkeley have devised software for planes’ on-board computers to stop pilots flying into protected areas, New Scientist magazine said today.

The system, dubbed “soft walls”, would resist a pilot trying to head towards a no-fly zone. If, for example, the pilot tried to steer into a zone on the left, the system would counter by banking right, gently at first and becoming more forceful if needed.

To the pilot, it would feel like fighting a strong wind. “When you reach a certain critical point, the pilot is banking as hard to the left as the aircraft will go, and that is only cancelling the force, so the aircraft is going straight,” Berkeley’s Edward Lee said.

“Soft walls” relies on the global satellite positioning system and includes an on-board database of GPS coordinates for no-fly zones. If the computer sensed an attempt to jam GPS signals it would switch to another system such as aircraft beacons.

Other anti-hijack systems usually involve intervention from the ground, such as ground control taking over the plane or automatic systems guiding it to safety. Lee said their disadvantage was that they relied on radio links with the ground, which could be jammed or hacked into.

Lee was working with U.S. aircraft giant Boeing Co. to develop the system further, New Scientist said.